Polymeric nanofibers are an emerging class of building block materials with applications in tissue engineering, filtration and nanocomposites. They are fabricated by a number of different processes, all of which can produce significant batch to batch differences. These fabrication processes include drawing, template synthesis, and electrospinning. Slight variations in the fabrication conditions and the large surface-to-volume ratio of individual nanofibers can produce batch to batch variations in the mechanical properties of the nanofibers that are produced.
The batch to batch variation greatly limits end uses of the nanofibers, which are often expected to perform critical functions in their end uses. Characterization of the mechanical properties of a batch of nanofibers can provide the necessary assurance that a particular batch of nanofibers is suited for their end uses. Determining nanofiber mechanical properties is not a straightforward task, primarily due to their very small dimensions and fragility. Practical testing methods and apparatuses that fit within the nanofiber manufacturing scheme are lacking.
Tension tests are best suited to testing of polymeric nanofibers because the fibers can only bear tensile forces well. Compared to other methods, such as nanoindentation and bending, tension tests require very few assumptions to extract mechanical properties, and tension tests allow for the application of a range of strain rates including creep and stress relaxation. With suitable tension testing schemes and devices lacking, attempts have been made to use other tests, e.g., nanoindentation, bending, and resonance frequency measurements, to obtain nanofiber mechanical properties. At best, these techniques provide results that are qualitative. Such testing techniques also fail to conform to standard set by the American Society for Testing and Materials) (ASTM). The ASTM approved method of evaluating the mechanical behavior of materials is tension testing.
Tension testing of nanofibers, nanotubes and nanowires has generally been conducted with the assistance of Atomic Force Microscopy (AFM) cantilevers and examined with a scanning electron microscope (SEM). A common setup involves one AFM cantilever or two opposing AFM cantilevers for mounting nanofibers. The AFM cantilever(s) serve the dual role of a load-measuring element and displacement sensor. A load is applied by deflection of the AFM cantilever, typically inside an SEM, which records the deflection of the AFM cantilever. Strain in the nanofiber/nanowire is measured by monitoring variations in the distance between the AFM cantilever tip and the substrate, or by tracking distinct features on the sample during the test. The force in the sample is calculated by multiplying the deflection and bending stiffness of the AFM cantilever.
Both the AFM cantilever and the SEM microscope place substantial limits on such tension testing techniques. Since the AFM is used to create displacement and as the force sensor, relative error is increasing when the stiffness of the cantilever is substantially different than that of the nanofibers being tested. Using an SEM is cumbersome, and use of an optical microscope is difficult due to the limited depth of field and the associated out-of-plane motion of the AFM cantilevers.
Some all-MEMS (micromechanical systems) test cells have been proposed as such platforms that can be fabricated in large quantities and at low cost. On-chip actuators are several times smaller than external actuators, and they fit conveniently inside an electron microscope. These devices have also had a limited range of force and displacement and are, therefore, poorly suited for use with strong and/or elastic nanofibers, some of which stretch by several tens of microns.
As suitable tension tests have been lacking for such fibers, different approaches have been attempted. These include nanoindentation, bending tests, resonance frequency measurements, and microscale tension tests. Nanoindentation lacks great accuracy due to uncertainties regarding the nanoindenter tip shape and the relative tip-fiber configuration, the effect of fiber surface curvature and roughness, and the adhesion force between the sample and the indenter. The local measurement provided by indentation is a poor predictor of the behavior of nanofibers during axial stretching, which is of paramount concern in many nanofiber applications. Three-point-bending and cantilever bending tests provide some information about nanofiber/nanowire elastic response and yield point, but the precise definition of boundary conditions at the nanofiber scale is unknown. These types of tests also fail to account for fiber sliding and rotation that are likely to occur at points of contact with the nanofiber during testing. Resonance frequency measurements have been successful for stiff metallic and ceramic nanowires, but polymeric fibers, with their high elasticity make extension of this technique non trivial since polymeric fibers can exhibit a whipping motion under the lateral excitation applied in such tests. The resonance of an AFM cantilever has also been used to calculate fiber stiffness, but this approach is similarly limited for highly elastic fibers.
Thus, efforts continue to be directed toward developing a useful tension test for highly deformable and or strong nanofibers and nanowires. Most efforts have used MEMS actuators that have limited range of motion, and many have utilized SEM for imaging, which have the drawback discussed above. Recently, Samuel et al reported on mechanical testing of pyrolyzed polymer nanofibers with a microdevice that included a leaf-spring loadcell that was actuated externally with a piezomotor. Samuel B A, Hague M A, Yi B, Rajagopalan R and Foley H C 2007 J. nanotechnology 18 1-8. An SEM was used for high magnification imaging to measure displacements. The displacement of a point (pixel) is measured in this technique, limiting the resolution to the pixel level. The speed of measurement with the technique reported by Samuel et al requires minutes to collect a datum point in a stress-strain curve, making their approach inappropriate for testing materials that are subjected to creep and stress relaxation, such as polymers and metals. In addition, the range of fiber deformation by the method of Samuel et al is limited to a few microns.